1. Field of the Invention
This invention relates to polyurethane elastomers plasticized and softened with alpha hydroxy acid esters of hydroxy polyols and methods of improving the plasticity of polyurethane elastomers by incorporation of a sufficient amount of such esters in the elastomer to modify the physical properties thereof.
2. Description of the Prior Art
Polyurethanes are obtained by the reaction of an ioscyanate with materials containing hydroxyl groups, e.g., polyesters, polyethers, castor oil, or simple glycols. These polymers may also contain urea groups resulting from the isocyanate reactions with amines or water.
In general, diisocyanates are used, that is compounds having two isocyanate groups: O.dbd.C.dbd.N--R--N.dbd.C.dbd.O. Most commercial urethane elastomers are based upon tolylene diisocyanate (TDI), methylene diphenyl isocyanate (MDI), or polymeric isocyanates (PMDI).
The polyol components are usually polyether or polyester based. Polyesters, such as polyethylene adipate or phthalate, are used and polyethers are low molecular weight macroglycols, usually based on polyalkylene oxides. ##STR1##
Because of the variety of raw materials which can be utilized to form urethanes, the polymers can be well suited for an individual application. In formulating products for these specific applications, additives can be used to improve specific properties. Plasticizers are employed to adjust the final hardness as well as the physical properties of the elastomer.
There are many plasticizing agents known to the art of polyurethane formulations. A plasticizer, by its nature, will soften a polymeric composition. In a polyurethane compound, a plasticizer can reduce hardness with resulting (1) lower tensile strength, (2) lower modulus values, (3) decreased tear strengths, and (4) improved elongation. Since the physical properties of a single polyurethane composition can be adjusted by selective plasticizer addition, stocking of fewer materials for a variety of applications is possible.
"Elastomers" from polyurethane resins as used herein may be defined as having elastic properties similar to those of vulcanized rubbers, both hard and soft, resist permanent deformation and have characteristics permitting use in the production of self-sustaining three-dimensional objects. Solid polyurethane elastomers can be milled, extruded, injection molded, calendared, cast, or otherwise processed into self-sustaining three-dimensional objects. Thus, elastomers are to be distinguished from films for coatings. The Condensed Chemical Dictionary, 10th Edition, Van Nostrand Reinhold Company, defines a film as being an extremely thin continuous sheet of substrate having a thickness no more than about 0.01 inch. A coating in the same publication is defined as a film or thin layer (assumedly of similar thickness) applied to a base substrate.
Various patents and publications over a long period of years have described a number of different alpha hydroxy acid esters of hydroxy polyols, but none are known to have suggested incorporation of these esters in polyurethane elastomers to plasticize and soften the resin formulation.